Acid rock drainage (ARD) is a natural process that occurs when sulfur-containing compounds in rock are exposed to air and water. When this process occurs in the context of mining operations, where sulfur-containing rocks are exposed as a result of open pit or underground mining, the process, and the acidic water it produces, is referred to as acid mine drainage (AMD). The process produces acidic waters as a result of the oxidation of the minerals pyrite (FeS2) and pyrotite (FeS) and other sulfur-containing compounds, generating sulfuric acid. The pH of the acidic waters is typically about 2.1 to 3.5. This low pH causes the water to leach metals from the rock and soils in contact therewith. Other mine wastewaters resulting from the operation of a mine, whether an underground or an open pit mine, including the water used in the operating process of the mine and from mill clean-up, are also often highly acidic. All such wastewaters, including ARD and AMD, are collectively referred to in this specification as “acidic mine wastewaters” (AMW).
The contamination of water supplies by metals in acidic mine wastewaters is a serious environmental concern. For example, the metals dissolved in such waters may kill fish and other aquatic life, and may pose human health hazards when they find their way into drinking water supplies.
One known method for treating AMW, using lime to precipitate metals as hydroxides, is the high density sludge (HDS) process. In this process, an excess amount of lime is applied to neutralize the acidity of the water and raise the pH to about 9-10. An appropriate flocculent is then added and the mixture is transferred to a clarifier, from which clean effluent is decanted from a sludge. There are a number of disadvantages with this process. It uses large amounts of lime, is very time-consuming, and requires handling a large quantity of sludge and further treatment of the sludge to stabilize it because metal hydroxides in the sludge tend to resolubilize if the pH changes. The process also requires expensive equipment to handle lime slurry. It frequently requires modification for removal of various metals to meet local environmental regulations. The lime also tends to become coated by metal precipitates at high metal contaminant levels, increasing lime consumption and therefore cost.
Kuyucak et al. (U.S. Pat. No. 5,427,691) discloses a modification of the HDS process to neutralize ARD in a two-stage process employing two reactors, instead of one-stage neutralization with lime and recycled sludge in the HDS process. The patent states that in the first reactor, a mixture of lime slurry and recycled sludge raises the pH of ARD to 4-4.5 to precipitate ferric hydroxide only. Then, the precipitates of ferric hydroxide can act as nuclei to promote crystallization in the second reactor where the pH of water is raised to 9-10 with lime and recycled sludge, under aeration. The generated sludge is separated from effluent at pH 9-10. Some portions of sludge are recycled and the remaining parts are disposed of. However, this improvement does not change the essential properties of sludge, nor reduce lime consumption significantly.
Zhuang (US Patent Publication 2004/0094484-A1, dated May 20, 2004) discloses a method using lignin derivatives to treat AMW. The combination and dispersion properties of lignosulfonates are utilized in this method, which may be referred to as the “lignin alkali coagulant method” (LACM), to combine metals as colloids for protecting lime from developing an external coating, resist precipitate fouling and scale formation, lubricate the smooth flow of both liquid and solid wastes, promote sludge flocculation and coagulation, and benefit the stability of the sludge. In treating AMW containing high levels of metals, the use of small amounts of lignosulfonates results in less lime consumption and a better quality of treated water, in comparison with conventional lime neutralization treatment (CLNT). However, the reduction in lime consumption achieved by this process is limited.
New technologies are needed to decrease sludge volume, improve the long-term stability of sludge under seasonal conditions, and minimize the lime coating problem and reduce lime consumption.